The height of the chassis for some makes of primer-movers differ from other makes, whilst the height of trailers also varies. It is also common practice for prime-movers to be coupled to a variety of trailers over a period of time in mixed fleet situations, rather than there being a continuing prime-mover/trailer combination. With such operating requirements the use of static fifth wheels will not ensure a relatively level load supporting surface on the trailer.
In the past mechanisms to enable adjustment of the height of the fifth wheel to deal with the above problems have necessitated hydraulic and pneumatic lifting arrangements all of which are costly.